This invention relates to improvements in activated-sludge sewage treatment systems. The invention further relates to an activated-sludge sewage treatment system with a hybrid aeration chamber comprising a primary, complete mixing section and a secondary, hydraulic plug-flow section. The invention especially relates to an activated-sludge sewage treatment system which provides treated effluent having a biological oxygen demand ("BOD") of approximately five parts per million ("ppm") without tertiary treatment.
Sewage treatment processes reduce undesirable or offensive waste from water. Primary sewage treatment removes solids from the water by using screens, grit chambers, skimming tanks and sedimentation basins. Secondary sewage treatment generally is preceded by primary treatment. It is a process whereby a biological treatment system rapidly breaks down organic material. Tertiary sewage treatment follows secondary treatment, and treats the effluent of the secondary treatment to further reduce the organic material from the water.
The activated-sludge sewage treatment system is a commonly used form of secondary treatment. It uses biologically active growths as a means to process raw sewage into relatively clean water. This microbiological culture is mixed with raw sewage (or the effluent of a primary clarifier) in a basin or chamber. Aeration means supply sufficient air to promote consumption of the colloidal and soluble organic matter (i.e. biologically degradable waste) in the sewage by the culture. When the microbes feed upon the organic matter in the sewage, they generate a biological mass of microorganisms (referred to as "activated sludge"), along with carbon dioxide, water, nitrogen compounds and traces of other components When substantially all of the colloidal and soluble organic matter has been converted into insoluble microbes and innocuous by-products, the mixture is directed to a clarifier, or secondary settling tank, which separates the relatively clean water, or finally treated effluent, from the microbes and allows the clean water to be decanted. The finally treated effluent is then released into a river or intermittent stream. A substantial portion of the activated sludge is recycled to the aeration basin, while a portion of sludge is continuously withdrawn to avoid excessive accumulation of recycled sludge.
For this system to produce a good quality of treated sewage, the decantation step must remove more than 99% of the solids from the feed mixture. Occasionally the microbiological growth produces a filamentous mycelium which settles very slowly, if at all. Filamentous mycelia in the effluent of the aeration chamber make it impossible to get a good quality of treated sewage from the decanter (clarifier). This filamentous growth is caused by various factors, but most often by too much or too little air. Penury dictates that if there is an inbalance of oxygen demand and oxygen supply, the error will almost always be a short oxygen supply. Once a filamentous growth starts, it is difficult to suppress. In a large aeration basin with an adequate air supply, it is possible to have localized areas of oxygen starvation which invite filamentous growth. The designing engineer must avoid this pitfall.
Traditionally the aeration basin has been a long, narrow chamber designed to promote plug hydraulic flow. Typical dimensions are from 20 feet by 200 feet to 40 feet by 1,000 feet, with a water depth of 12 feet to 18 feet. For economy of land use and of construction costs, the longer chambers are usually built in three parallel sections with a common wall between sections. The plug hydraulic flow of the mixed liquor through the aeration basin insures the maximum reduction of pollutants in the clarified effluent, while maintaining a high rate of oxygen usage throughout most of the chamber volume. In fact, one of the problems with the plugflow aeration basin is the tendency to grow filamentous mycelia in spots of localized oxygen starvation.
In 1980 in Suwa City, Japan, a shake-down operation was conducted in a new, partially-completed sewage-treatment plant. An aeration chamber 5 meters wide, 5 meters deep, and 60 meters long (operating in a plug-flow hydraulic mode) was treating some 8,000 metric tons per day of domestic sewage with 10 ppm to 80 ppm of BOD, and consistently producing an effluent containing 3 ppm BOD and 7 ppm of suspended solids. In this test run, the recycle rate was 100% (the volume of water with activated sludge returned to the inlet of the aeration basin was equal to the volume of the treated, clarified water discharged to the public stream). There was de facto flow equalization, and uniform aeration was used the length of the basin to give 3 ppm of dissolved oxygen at the discharge end. By calculation it is seen that the rate of BOD dissipation was some 15 lbs per day per 1,000 cubic feet of aeration chamber volume, which would make this operation too costly for most municipalities.
Within the past thirty years, an alternate type of aeration basin, the complete-mix system, has come into use. In this aeration basin, the incoming sewage and recycle sludge are rapidly mixed with a large volume of partially-treated sewage, and the entire contents of the chamber are mixed continually. In this way all mycelial growth occurs in a liquor with a BOD little higher than that of the treated effluent, and it is easy to avoid filamentous growth due to localized oxygen starvation. The claimed advantages of the complete-mix over the plug-flow aeration chamber are:
1. Lower cost of construction per unit volume of aeration chamber.
2. Ease of design and operation to avoid local areas of oxygen starvation (which promote the growth of filamentous mycelia).
3. Nitrification and denitrification occur simultaneously in the complete-mix basin, thereby reducing the amount of nitrogen compounds in the plant effluent.
4. Ease of adding more aeration capacity to cope with increased loading.
The disadvantage of the complete-mix aertion chamber is that, since the mycelial growth occurs in a medium with the same level of BOD as that of the clarified effluent, any attempt to improve the quality of the effluent will drastically reduce the volumetric capacity to remove BOD.
Calculations of sewage-treatment plant capabilities are usually based on an average daily volume of sewage flow and an average content of organic pollutants, expressed as biological oxygen demand (BOD). Unfortunately, the inflow rate to a sewage-treatment plant usually varies widely over a 24-hour period. Each sewage-collection system will have its own characteristic diurnal flow pattern, but most municipal systems will have a flow pattern similar to that shown in FIG. 1. From FIG. 1, it is seen that the maximum hourly flow rate is twice the average flow rate, and the minimum hourly flow rate is half the average flow rate. Also, at periods of high flow rate, the BOD concentration of the sewage is usually higher than average. Thus, a system with a flow of 10,000,000 gallons per day of sewage with an average BOD of 200 ppm can be expected to have a maximum flow of some 800,000 gallons per hour with a BOD of 300 ppm, and a minimum flow of some 200,000 gallons per hour with a BOD of 150 ppm.
Various authorities in the field of sewage treatment have pointed out the advantage of making a wide spot in the sewage-collection system in order to provide a more constant inflow rate to the sewage-treatment plant, but neither flow-equalization nor load-equalization to the sewage-treatment plant is considered to be cost effective, and neither is often used. In a sewage-treatment plant with two or more aeration chambers operating in parallel it is not practical to put a chamber into operation and take a chamber out of service every day in order to accommodate the varying load. Some years ago there was a move to have the effluent of the aeration basin discharge over a narrow weir, so that an increased flow into the basin would raise the surface level in the basin and, due to the increased submergence of the impellers of pier-mounted surface aerators, would cause more oxygen to be absorbed by the chamber contents. This system has not been widely adopted.
In many activated-sludge sewage treatment plants in the United States, the air flow to the aeration chambers is relatively constant over a 24-hour period, and there is minimum instrumentation to adapt the air-flow rate to the BOD load. During the hours of low BOD inflow, the extra oxygen is adsorbed by the activated sludge, and during the hours of high BOD inflow, oxygen is desorbed from the sludge to supplement that being introduced by the aeration equipment. If the sewage-treatment plant has a diurnal flow pattern similar to that shown in FIG. 1, with adequate activated sludge (perhaps equal to 15 or 20 days' new growth) in the aeration basin, and has a dissolved oxygen content in the mixed liquor of 2 ppm at 9:00 a.m., the activated sludge will have enough adsorbed oxygen to supply the peak oxygen demand for biological growth. Sewage-treatment plants which use this system for controlling air flow tend to have a treated-sewage effluent with a BOD which varies widely during the day. Hopefully the average BOD for the day will meet the standards set by the U.S. Environmental Protection Agency.
In recent years there has been a trend to use computer control to better match the aeration rate to the hourly inflow rate of BOD. There is always a question as to whether the reduction of power costs for aeration and/or the improved quality of the treated effluent justify the additional aeration capacity to meet the peak load, the cost of the control equipment, and the high maintenance cost of the control system.